US farmland occupies 895 million acres (3.6M km^2) [1]. Average solar irradiance on most US farmland is about 200 W/m^2 [2]. By being generous and assuming vertical farming is 100x more efficient than natural photosynthesis, the energy requirements come out to 7.2 terawatts of generating capacity. In contrast, the US currently has 1.1 terawatts of generating capacity. [3] Vertical farming isn't just uneconomical: it's completely absurd.
Thanks for doing the numbers, it’s interesting that in 50 years people are predicting annual TW installs of global solar capacity. So maybe a technology for the future if storage and distribution gets solved.
Except none of that matters, Solar Irradiance has very little to do with actual photosynthesis. You are measuring Watt's as a pure energy source, that's useless in this case. Photosynthetic photon flux density (PPFD's) are what's important, that's the amount of light wavelengths that the plant needs to absorb to power photosynthesis. Here is a definition from another source, I'll post the link at the bottom:
PPFD is a measure of photosynthetic active radiation, PAR for short. PAR is not a measure of anything itself but is more of a description. PAR light is all the visible wavelengths of light which cause photosynthesis, found within the 400-700 nanometer range. PPFD is a ‘spot’ measurement that tells you how many photons from the PAR range hit a specific area of your canopy over time. It is expressed as micro moles per square meter per second (μmol/m2/s). For this reason, PPFD is the most accurate measure of light power. First, unlike other measures, it considers the entire spectrum of light that plants see. PPFD also takes into account the amount of light that will actually reach the plant instead of focusing only on the point of origin. A light source can be very bright and powerful, but if it is too away from the plant, or obstructed in some way, the plant won’t be getting all the light it needs for photosynthesis. PPFD controls for this kind of inaccuracy.
With LED's you can place the plants much closer to the light source and control the color spectrum along with the obstructions 24/7. It allows you to grow indoors at an extremely efficient rate. Your formula has almost nothing to do with indoor farming.
Quick summary: PPFD measures the amount of light wavelengths that the plant captures that kicks off the chemical reaction of Chlorophyll. Red and Blue lights are the vast majority of wavelengths that interact with Chlorophyll. That's a major reason why the solar irradiance measure of pure Wattage is irrelevant.
>Except none of that matters, Solar Irradiance has very little to do with actual photosynthesis. You are measuring Watt's as a pure energy source, that's useless in this case
>That's a major reason why the solar irradiance measure of pure Wattage is irrelevant
You are quite literally missing the tree for the forest. Parent assumes that photosynthesis is 1% efficient with sunlight so he added the 100 factor to estimate how much energy plants would need if photosynthesis was 100% efficient and you could skip photosynthesis just feed plants energy straight from the power grid and on top of that he is assuming you can turn 100% of solar radiance into electricity.
If we assume 20% efficient solar panels your growing lights need to be 500 times more effective or power efficient than sunlight.
You should have based your argument on arguing that the vast majority of acres that the parent used are e.g. suitable for solar panels but not for farming plants and hence the number of acres is unfairly bloated. But all I see is excuses in trying to tell people how your growing lights are going to be 500 times more efficient when we know that isn't going to happen.
Are you able to achieve a 10000x efficiency improvement vs natural photosynthesis by using those technologies? If not, the idea is still completely unfeasible.
IT'S COMPLETELY IRRELEVENT, the efficiency at generating electricity is not what powers photosynthesis. It's a chemical reaction to light wavelengths (ie the absorbing of Red and Blue light) that powers it. More watt's doesn't equal more growth, it equals more heat which allows longer growing seasons. LED's and Sunlight have EXACTLY the same level of photosynthesis, none of the math you did matters at all. For LED's you need to calculate coverage of the area not pure power. If you produce Red or Blue wavelength's that's all that matters, the Sun does that and a hell of a lot more but we don't need the power of the sun or even a tiny faction of it's power for photosynthesis, we need to heat and cool and creat weather which effect plant growth but Watt's is irrelevant.
>For LED's you need to calculate coverage of the area not pure power
He already abstracted over that by using efficiency.
>none of the math you did matters at all.
One would have expected an increase in coverage to increase efficiency by 10 vs sunlight which would have require explaining another factor 10 which could have been explained by e.g. saying that converting useless wavelengths into useful wavelengths increased efficiency by 10.
You're arguing that the things you have said do not increase efficiency. That is even more damning than a ridiculous efficiency gain.
WHAT MATH?!?!? Electricity doesn't have any thing to do with it except in very tiny amounts. You need light coverage not heat wattage! There is no math to do! Photosynthesis doesn't work like that. It's a chemical reaction to light wavelengths. I posted numerous papers detailing this. The unit of measure is PPFD Photosynthesis Photon Flux Density, that's the coverage of the number of light wavelengths the plant can absorb. LED's provide plenty of PPFD's, exactly the same as the Sun. So all you need is the correct amount to cover your growing space with a minimum amount.
[1] https://www.statista.com/statistics/196104/total-area-of-lan...
[2] https://en.wikipedia.org/wiki/Solar_irradiance
[3] https://www.eia.gov/energyexplained/electricity/electricity-...